FormBuilder initially sets unresolved children as NULL, until resolved.
If FormBuilder::remove() is called before that child is resolved, the
if statement turns false, because isset(null) is false, when it should
be true. Instead, we should check to see if the key exists, and if so,
process and unset it.
Closes#4803
In order to perform some introspection on a FormBuilder instance,
we need to be able to get its children. Almost everything is accessible
in this class, but the children are not.
This PR adds a all() method to respect the current API (get(), remove(),
...).
